1. Field of the Invention
The present invention relates generally to the translation of addresses across networks and more specifically to application layer gateways.
2. Description of the Related Art
Machines must have unique addresses if they are to communicate with each other over a network, such as the Internet. IPv4, the current version of the Internet protocol, provides 32 bits for the IP address. Although 32 bits theoretically allows 4,294,967,296 unique IP addresses, addresses are separated into classes and set aside for multicasting, testing and other special uses. The actual number of available addresses from Network Solutions, Inc., the entity that provides registration services for the Internet, are between 3.2 and 3.3 billion. With the explosion of the Internet, address depletion is a serious concern. Additionally, many enterprises want to manage their own networks, and do not want to rely on Network Solutions, Inc. for the addressing of every machine on their networks.
One solution for conserving addresses is referred to as network address translation (NAT). NAT allows an intermediary device (e.g., computer, router or switch) to act as an agent between a public network (e.g., the Internet) and a private local network. Each device on the local network is assigned a local IP address that is used only for local communications. Additionally, the local network is given a range of public IP addresses to represent the devices on the local network when communicating with the public network.
When a local computer attempts to communicate with a computer outside the local network, the intermediary device (NAT box) maps the local computer's IP address to one of the public network's IP addresses and replaces the local computer's address with public network's IP address. If the local network has more devices than public IP addresses, the public IP addresses must be shared among the local computers.
A feature of IP is multiplexing, which allows a computer to maintain several concurrent connections with remote computers using different TCP or UDP ports. A NAT box takes advantage of this feature by modifying the TCP/UDP ports for the traffic from the local computers such that it can uniquely map the local address and local TCP/UDP port number (tuple) to the public IP address and assigned TCP/UDP port number. Therefore, when a NAT box uses network address port translation (NAPT), it does not need the same number of public addresses as local devices. It should be noted that NAPT is included in the definition of NAT.
In addition, particular applications (e.g., DNS (domain name service), FTP (file transfer protocol), H.225/H.245) may embed IP addresses and/or TCP/UDP ports in the payload for various application specific purposes. NAT boxes are given application-specific knowledge in order to translate such addresses, as described in the Internet Engineering Task Force's RFC 2663, “IP Network Address Translator (NAT) Terminology and Considerations,” by P. Srisuresh and M. Holdrege (August 1999), incorporated herein by reference in its entirety and for all purposes.